Various types of micro optical beam shaping elements are sometimes incorporated into optical or optoelectronic modules such as cameras and other imagers, LED arrays, proximity sensors and other optical sensors. The beam shaping elements (e.g., lenses) often are formed of a plastic material, which can be significantly less expensive than forming the beam shaping elements of glass.
Some applications require environmentally stable materials with good optical properties that can be formed on mechanically rigid substrates. Individual lenses or lens arrays, for example, sometimes are formed or bonded directly on a substrate that has a lower coefficient of thermal expansion (CTE) than the plastic lenses. Consequently, the lens is mechanically constrained at the interface with the substrate. At elevated temperatures, this mechanical constraint and the difference in CTE between the two materials can give rise to deleterious shear stresses in the higher-CTE plastic lens. These shear stresses engender significant dimensional distortions in the plastic lens resulting in a loss or severe degradation of optical function. For example, during operation, the temperature of an optoelectronic module (comprising a high-CTE lens bonded directly to a low-CTE substrate) may increase, sometimes significantly. At elevated temperatures the mechanical constraint combined with the different CTE would distort the plastic lens. For example, a dimensional change in the lens in the direction of the optical axis (i.e., in the direction perpendicular to the surface of the substrate to which the lens is attached) can be relatively large and, consequently, the lens focal length can change significantly as a function of temperature. The resulting optical effect or image can become degraded or distorted.